In recent years, the fluorescent lamp, which requires less energy than the incandescent lamp to produce the same amount of light, has enjoyed increasing popularity. In many modern offices, fluorescent lamps are used to the complete exclusion of incandescent lamps. Other gas discharge lamps, such as sodium-vapor lamps, have replaced incandescent lamps in outdoor lighting applications.
To maintain high energy efficiency, reliable operation, and long lamp life, these gas discharge lamps may be operated in conjunction with a resonant inverter ballast circuit, such as the ballast shown in the inventor's U.S. Pat. No. 4,933,605.
Electronic dimming control circuits, such as the circuit disclosed in the inventor's copending U.S. patent application Ser. No. 07/410,480 filed Sep. 21, 1989, have been used with resonant inverter ballasts to provide effective low-voltage control of gas discharge lamp brightness. In the preferred embodiment of the dimming circuit disclosed in the Ser. No. 07/410,480 application, the dimming level is controlled by a low voltage input level produced by integrating a variable pulse width output from an electronic dimming control circuit.
Such electronic dimming circuits are generally provided with an operator-adjusted manual control for setting the desired level of gas discharge lamp luminosity. It is also known to turn lamps on and off in response to photocell measurement of ambient light levels. In a common application of this technique, a photocell may be used to turn on a parking lot lamp during periods of darkness (i.e. night) and to turn the lamp off during periods when sufficient external light sources (such as sunlight) are available, thus conserving energy.
In an office setting, each work area must at all times be provided with at least a minimum level of light. The minimum necessary light level is determined based on the tasks performed in the area. Fluorescent lamps are generally installed in size and number sufficient to provide the minimum required light level in an area under the assumption that no other light sources will be available. A dimming circuit may be provided to adjust the light output of the lamps, permitting multiple uses of the area and compensation for changes in external light.
At times, other light sources are also operating in the area so that the amount of light produced is more than is needed, and the operation of the lamps at the same intensity used in the absence of other light sources is a waste of energy. For example, during the day sunlight may enter through windows and skylights. When these other light sources are available, the preset brightness of the gas discharge lamps will not be needed in its entirety since the external light source provides some or all of the minimum needed light in the area. It would be possible to conserve large quantities of energy, possibly up to 30% of the energy used to light a typical office building, if the light output of gas discharge lamps could be limited at all times to the minimum required level.
Additionally, in the workplace, it is usually desirable to have a constant level of light on work surfaces. Continually changing light levels result in periods of glare when too much light is provided and period of increased difficulty in resolving images when too little light is provided. A worker's eyes must adjust to resolve images at a given light level. Thus, continual light level variations requires continuous optic compensation, and this eyestrain over time can adversely affect health and productivity.
U.S. Pat. Nos. 4,482,844 to Schweer et al., and 4,371,812 and 4,394,603 to Widmayer, show systems for dimming a fluorescent lamp in response to ambient light conditions. U.S. Pat. No. 4,464,606 to Kane discloses a fluorescent lamp dimmer with an electronic inverter that is controlled in response to signals from a ceiling-mounted ambient light sensor. The dimming control circuit shown operates using low voltages and pulse width modulation of the power to the lamps, but does not integrate pulse-width modulated control signals to produce the dimming control signal that controls the width of the lamp switching control pulses. As far as the inventor is aware, electronic dimming control circuits of the type disclosed in the aforementioned pending application have not been equipped with circuits for adjusting the lamp output to minimize energy consumption while maintaining a constant light level in an area.